drm/i915: Remove manual breadcumb counting

Now that we know we measure the size of the engine->emit_breadcrumb()
correctly, we can remove the previous manual counting.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125120005.25191-1-chris@chris-wilson.co.uk